1. Field of the Invention
The present invention relates to a digital signal recording and reproducing apparatus capable of recording and reproducing a digital information signal, such as a digital video signal having compressed data and reduced bandwidth.
2. Description of the Prior Art
A digital signal recording and reproducing apparatus, such as a digital video tape recorder (e.g. a digital VCR), for recording and reproducing digitized video signals has been developed for professional use because it has reduced in deterioration of recorded and reproduced video signals caused by dubbing as compared with an analog signal recording and reproducing apparatus. Digitized video data, however, large contain an amount in the quantity of information, and consume a very large amount of tape for recording. So, for consumer-use, attempts have been made to record the digital video signal after data compression and bandwidth reduction by predictive coding, DCT (discrete cosine transform) coding, and variable length coding. For example, the data in each frame of the video signal is compressed, divided and recorded on plural consecutive tracks of a tape (e.g. "An Experimental Digital VCR with 40 mm Drum, Single Actuator and DCT-Based Bit-Rate Reduction", S. M. C. Borgers et al., IEEE Transactions on Consumer Electronics, Vol. 34, No. 3, pp. 597-605, August 1988).
This method is explained below with reference to FIG. 1. A magnetic tape 11 having a specific tape width is wound on a cylinder 12, and is driven to run at a constant speed. In recording, an input video signal 211 is inputted to an A/D (analog-to-digital) converter 201 to be digitized into a digital video signal 212 having a specific bit width. A compressor 202 compresses the data of the digital video signal 212, and produces a compressed video signal 213. At this time, the compressor 202 divides a specified group unit (one frame) of the digital video signal 212 further into small blocks, and shuffles the small blocks. A modulator 203 modulates the compressed video signal 213 to be suited to magnetic recording, and produces a modulated video signal 214. The modulator 203 also combines the modulated video signal 214 with a pilot signal when necessary. A switch 204 is connected to the 204A side when recording to send the modulated video signal 214 to magnetic heads 13A and 13B and record the signal on a magnetic tape 11. By the shuffling conducted in the compressor 202, the video signal within each specified group unit (in each frame) is divided into six tracks.
When reproducing and magnetic heads 13A, 13B mounted on the cylinder 12 reproduce the modulated video signal 215 from the magnetic tape 11. The reproduced modulated video signal 215 is demodulated into a compressed video signal 216 by a demodulator 205, and then decompressed into a digital video signal 217 by a decompressor 206. A D/A (digital-to-analog) converter 207 converts the digital video signal 217 to an analog video signal 218.
In such digital recording and reproducing apparatus (for example, a digital VCR), when a new video signal is recorded on an already recorded magnetic tape, the linkage part of the recorded tracks and the shuffling group units partially become discontinuous. As a result, when reproducing a discontinuously recorded tape, noise is formed in the reproduced image at this discontinuous portion, and the quality of the reproduced image is impaired.
For example, as shown in FIG. 2, when a new signal is newly recorded in a pre-recorded region, the continuity of the tracks is interrupted at the starting position of the new recording. Further, when one frame which is the group unit of shuffling and compression is divided into six tracks to be recorded, the starting position of the new recording also spoils the continuity of this six-track unit. Therefore fore, when such a discontinuously recorded tape is reproduced, the tracking control is disturbed due to track discontinuity, or the decompression operation is disturbed due to discontinuity of the group unit, thereby producing noise temporarily on the reproduced screen.
To avoid such discontinuity, for example, a method of eliminating discontinuity by mechanical action is disclosed in U.S. patent application "Reproducing Apparatus", Yoshio Sakakibara et al., Ser. No. 08/041,124 (tentatively assigned), filed Mar. 31, 1993. According to this method, a magnetic tape is preparatory run in a reproduction in a pre-roll section before the start of recording, and the reproduced position of the magnetic tape 11 is changed by an amount corresponding to a specified number of tracks so that the phase of the frame of the reproduced signal coincides with the phase of the frame of the signal to be recorded. At the moment of reaching the recording start position, the actual recording is started. In such method, however, since it requires a mechanical action, it takes a very long time until the transfer action of the reproduced track position of the magnetic tape is finally stabilized.